Conventionally, there has been known a signal transmission system including: a local unit that transmits an analog image signal (i.e., an RGB signal) output from a computer, and a response signal to the operation of a keyboard and a mouse to a remote place with a Cat 5 (Category 5) cable including four pairs of signal lines; and a remote unit that transmits an operation signal of the keyboard and the mouse to the local unit and receives the analog image signal and the response signal (See Japanese Laid-Open Patent Publication No. 2004-356939).
In the above-mentioned signal transmission system, the local unit transmits the analog image signal to the remote unit with three pairs of signal lines, and transmits the response signal to the remote unit with a remaining pair of signal lines.
In the above-mentioned signal transmission system, when the local unit transmits an analog audio signal from the computer to the remote unit, the local unit converts the analog audio signal in digital form, and transmits three signals of a digital audio signal, a channel clock (LRCK), and a bit clock (BCK) to the remote unit with two pairs of signal lines in the Cat 5 cable. Therefore, when the local unit transmits the analog image signal and the analog audio signal to the remote unit, two Cat 5 cables connected between the local unit and the remote unit are necessary.